Dense wavelength division multiplexing (DWDM) requires optical spectrum analyzers (OSAs) that have higher spectral resolution than is typically available with current OSAs. For example, grating based OSAs and autocorrelation based OSAs encounter mechanical constraints, such as constraints on beam size and the scanning of optical path lengths, which limit the degree of resolution that can be obtained.
As an alternative to grating based and autocorrelation based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems. FIG. 1 is a depiction of a prior art optical heterodyne detection system. The optical heterodyne detection system includes an input signal 102, an input waveguide 104, a local oscillator signal 106, a local oscillator waveguide 108, an optical coupler 110, an output waveguide 118, an optical receiver 112, and a signal processor 116. The principles of operation of optical heterodyne detection systems are well known in the field of optical heterodyne detection and involve monitoring the heterodyne term that is generated when an input signal is combined with a local oscillator signal. Optical heterodyne detection systems are not limited by the mechanical constraints that limit the grating based and autocorrelation based OSAs. The spectral resolution of an optical heterodyne system is limited by the linewidth of the local oscillator signal, which can be several orders of magnitude narrower than the resolution of other OSAs.
In order to improve the performance of optical heterodyne detection systems with regard to parameters such as sensitivity and dynamic range, it is best to have a high signal to noise ratio for the heterodyne signal. However, the desired heterodyne signal coexists with other direct detection signals. The direct detection signals in DWDM systems include intensity noise from the input signal and shot noise from the local oscillator signal that can mask the desired heterodyne signal. One technique for improving the signal to noise ratio of the heterodyne signal involves reducing the intensity noise by utilizing two detectors to accomplish balanced detection. Although balanced detection is useful in improving the signal to noise ratio for the heterodyne signal, it has limitations.
Another technique for heterodyne signal detection described in U.S. Pat. No. 4,856,899 involves amplifying the input signal before the input signal is combined with the local oscillator signal in order to increase the amplitude of the heterodyne signal. Although amplifying the input signal increases the amplitude of the heterodyne signal, the amplification also increases the intensity noise of the input signal and may not improve the signal to noise ratio of the heterodyne signal.
In view of the prior art limitations, what is needed is an optical heterodyne detection system that generates a heterodyne signal with a high signal to noise ratio.